486 GEOLOGY OF THE BLACK HILLS. 



having silica as high as 82 per cent. The percentage of silica in these 

 rocks was determined, as well as the amount dissolved in hydrochloric acid. 

 The loss in acid, however, cannot be considered of very much value, o wing- 

 to the change in the constituents of the rocks from weathering, which 

 materially increases the proportion of the soluble part and causes it to 

 appear as if there were decomposable minerals present. Notwithstanding 

 this uncertainty, the results of both determinations are given further on in 

 the table on page 526. 



From the characteristics mentioned, the varieties of rhyolite are dis- 

 tinguished by Zirkel as follows : 



Crystalline-granular — of a granitic character. 



Felsitic — a felsitic groundmass, without separated crystals. 



Porphyritic — a felsitic or crystalline groundmass, with imbedded crystals. 



Half-glassy or glassy — consisting entirely, or in very large part, of glass, obsidian, 



pearlite, &c. 



Almost all of the Black Hills rhyolites belong to the porphyritic 

 variety, as they generally have large sanidin, biotite, and other crystals 

 porphyritic in a fine-grained groundmass. Other varieties occur in a few 

 instances, which will be mentioned in the detailed description of the thin sec- 

 tions under the microscope. 



Quartz is the most important of the crystalline minerals separated 

 from the groundmass. It occurs in grayish and transparent, round grains 

 or crystals, which have a distinct line of demarkation between them and 

 the groundmass. Macroscopically, they can be distinguished from the sani- 

 din crystals, which are in most cases also brilliant and glassy, by their 

 different luster and conchoidal fracture, while sanidin has a good cleavage 

 The hardness of the quartz assists in the distinction. It is sometimes crys- 

 tallized in the form of doubly terminated c^stals, with prismatic faces. In 

 the thin section under the microscope, the quartz appears either in rounded 

 grains or crystals. It can be distinguished from sanidin microscopically by 

 its greater pellucidity, polarizing in brilliant colors, and its lack of cleavage 

 lines. The crystalline form is also of great assistance, when the resemblance 

 between the two minerals makes the distinction doubtful. It often contains 

 microlites and brownish glass bodies, which latter inclusions are sometimes 

 hexagonal in shape. Characteristic for the quartz in trachytic rocks, as 



